We have isolated and partially purified a cell surface growth inhibitor from both mouse and bovine cerebral cortex cells. The inhibitory molecules are fucosylated glycopeptides that are released from cell surfaces by mild proteolysis. Several primary, tissue culture-adapted and transformed cell cultures have been used to illustrate the notion that malignant cells are relatively resistant, or completely refractory, to the action of the inhibitors. Although target cell gangliosides are important in the inhibitory action, the gangliosides may not be receptors for the glycopeptide inhibitors. Cell populations, arrested in the mitotic cycle by the action of the inhibitor molecules, seem to be arrested in the G2 phase. The inhibitory action is nontoxic and completely reversible, and nanogram quantities are effective in inhibiting both cell protein synthesis and cell division. The mouse and bovine glycopeptides appear to be antigenically related, and the glycopeptides have been purified over 6,000-fold by gel filtration, lectin affinity chromatography and isoelectric focusing on wide-\and narrow-range ampholyte gradients.